APPENDIX. CHAPTER XX. 313 



4. The frog (b) will have had its whole brain removed. Its heart 

 will continue to beat, but its breathing movements will cease, be- 

 cause the respiratory centre, which lies in the medulla oblongata, has 

 been cut away. It will also lie down squat, instead of sitting up 

 like a normal frog, because its most important muscle co-ordinating 

 centres have been removed with the mid-brain and cerebellum. Left 

 to itself the animal will, within half an hour of the removal of the 

 head, pull up its hind legs into their natural position, but after this 

 it will make no movement. It has no volition. 



5. Such a frog can, however, perform many co-ordinated reflex 

 actions, which may be illustrated as follows: (a) Pinch a toe; it will 

 be pulled away, (b) Soak some blotting-paper in vinegar, and 

 then cut the paper into small pieces about | inch square. Put these 

 bits of paper on different regions of the frog's skin, dipping the ani- 

 mal in clean water after each application, to wash away the vinegar. 

 It will be found that the brainless creature moves its limbs so as to 

 wipe away the acid paper placed on its skin. The frog without its 

 brain has no Will and no consciousness; but its spinal cord when ex- 

 cited by afferent nerves, whose ends the vinegar stimulates, excites 

 in turn efferent nerves which stimulate muscles, whose contrac- 

 tion produces a movement calculated to rub away the irritating 

 object. 



6. Now run a stout pin down the frog's neural canal so as to de- 

 stroy its spinal cord. It will be found that no subsequent pinching of 

 the creature or putting of vinegar on its skin causes any movement. 

 Its muscles and nerve-trunks are intact, but the spinal reflex centre, 

 which in the previous experiments was excited by afferent nerves, 

 and then in turn stimulated efferent nerves, is destroyed. The heart 

 continues to beat, on account of the automatic nerve-centres in it; 

 but no voluntary and no reflex actions are exhibited by the animal. 



7. The nerve-trunks and the muscles are, however, still active. 

 Turn the frog on its back and carefully expose (p. 299) the origins of 

 the sciatic nerves. On pinching these, the muscles of the leg will be 

 seen to contract. The irritation of the nerve by the pinch starts in 

 it a nervous impulse which travels down the nerve -branches to the 

 muscles. 



